1. Field of the Invention
This invention relates to a golf ball comprising a core and a cover having at least one thin spherical layer, and a method for manufacturing the same.
2. Prior Art
Solid golf balls constructed of a polybutadiene rubber core at the center and a spherical cover made of ionomer resin on the outside are in widespread use today. There has been a trend in recent years toward specific combinations of two or three layers in the cover, each made of a distinctive material. The aim is two-fold: to approach more closely a a level of performance which both incorporates such desirable features of thread-wound balls as their good spin receptivity and the easier control they allow over where the ball stops after it has been hit and also provides the longer carry of the solid balls, and to further increase the ball's carry. Moreover, within the limit imposed by regulations on the outside diameter of the ball, the core which generates most of the rebound energy that powers the ball's flight must have the largest volume possible. This set of circumstances has given rise to a need for thinner molded covers than in the past.
When solid golf balls first appeared, development began with two-piece balls made of a core and a one-layer cover. The cover was composed of a single layer of ionomer resin that was formed, separately from the core, into a pair of half-cups in a mold, then placed over the core and molded integrally with the core in another mold. For this reason, the thickness of the cover was generally set at about 2.0 mm. Later, covers came to be molded with injection molding machines by injecting resin about the periphery of a core positioned at the center of a spherical mold. Because the cover thickness was maintained at the preexisting value of about 2.0 mm, there was no need to make the injection molding machine fill speed particularly rapid. Thus, given a melt flow rate (MFR) in 9/10 min at 190.degree. C. of from 1 to 10 for the injection molding material, the standard practice has been to set the fill speed for molding one ball at from 10 to 20 cm.sup.3 /s.
However, as noted above, when the number of layers in the cover increases to two or more and it becomes necessary to reduce the thickness of individual layers, the injected layers of resin are thinner so that they cool more rapidly. At the above-indicated range of fill speed in the injection molding machine, curing begins to arise before the resin for a given layer has extended over the entire surface of the core. This raises the possibility that the thin layer will not have a uniform thickness over the peripheral surface of the ball.
Although attempts have been made to adjust the properties of the molding material and the mold fill speed by trial and error, not only is such adjustment difficult, excessive modification of these parameters sometimes leads to a decline in ball performance and an increase in the level of production defects.